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A thermodynamic process in which the system undergoing the change exchanges no heat with its surroundings. An increase in entropy or degree of disorder occurs during an irreversible adiabatic process. However, reversible adiabatic processes are isentropic; that is, they take place with no change in entropy. In an adiabatic process, compression always results in warming, and expansion always results in cooling. See Entropy, Isentropic process

During an adiabatic process, temperature changes are due to internal system fluctuations. For example, the events inside an engine cylinder are nearly adiabatic because the wide fluctuations in temperature take place rapidly, compared to the speed with which the cylinder surfaces can conduct heat. Similarly, fluid flow through a nozzle may be so rapid that negligible exchange of heat between fluid and nozzle takes place. The compressions and rarefactions of a sound wave are rapid enough to be considered adiabatic. See Sound, Thermodynamic processes

(ad-ee-ă-bat -ik) A process that takes place in a system with no heat transfer to or from the system. In general, a temperature change usually occurs in an adiabatic process.

a process, occurring in a physical system, in which no heat is exchanged with the surroundings. An adiabatic process can occur in a system surrounded by a heat-insulating (adiabatic) enclosure. One such example is the power stroke of a heat engine, where the gas or vapor expands in a cylinder with heat-insulating walls and piston, in the absence of irreversible conversion of friction into heat.

An adiabatic process can also be achieved in the absence of an adiabatic enclosure. In that case, the process must occur so rapidly that there is no time for any exchange of heat between the system and its surroundings. That happens, for example, when a gas is compressed by a shock wave, where the gas, having no time to give off the heat released, heats up vigorously. When the wave moves at a speed of about 1 km/sec (a speed attained by modern supersonic airplanes) and compresses the air fourfold, the air temperature rises to 700°C. Adiabatic expansion of the gas with the carrying out of work against external forces and the forces of mutual molecular attraction causes the gas to cool. This method of cooling gases is the basis of the gas liquefaction process. The adiabatic process of demagnetization of paramagnetic salts makes it possible to attain temperatures close to absolute zero.

An adiabatic process can be either reversible or irreversible. In the case of the reversible adiabatic process, the entropy of the system remains constant. A reversible adiabatic process is therefore called isoentropic. On a phase diagram of the system it is represented by an adiabatic curve or by an isoentropic curve. In an irreversible adiabatic process the entropy increases.